Q&A: Cracked Manufactured Stone

Q.Last spring I completed a house with
manufactured stone columns on the porches on both the north and
south sides. The columns on the south porch have cracked at the
corners (see photos), while the columns on the north porch are
fine. The cracks are only on the two outside corners of each
column, not the house side. The cracking occurred right away
and hasn’t gotten worse.

The deck is supported on sonotubes below frost and is all
pressure treated. The columns are built like the chimney
surround (which is also covered with stone and has no
problems), out of 2x4 studs and plates, covered with OSB. The
bottom third of each 2x4 “box” is covered with
eaves membrane, and the top is covered with felt. I used wire
lath, though I didn’t wrap the lath around the corners. I
have built many columns using the same methods, with no problem
before.
The columns support LVL beams across the top. Above the LVL
are monotrusses whose bottom chords also support the porch
ceilings. On the north side, the ceiling is 6 feet wide; the
south ceiling is 10 feet wide. The porch ceilings are
tongue-and-groove boards, with no venting.
Any ideas about why I’m getting these
cracks?

A.Steve Thomas, who worked for 15 years in the
stucco and masonry industries, responds: First, in my
opinion, you’ve done nothing wrong in not turning the
corner with your lath. I’m not familiar with your mix,
but the work looks very proper, given the photos you provided.
The north-facing porch (with a 6-foot bottom chord and no
“seasonal” sun hammering) as well as the chimney
escaped scot-free. So you have to look at conditions that exist
only on the south (sunny) side.

Your notes indicate that the stonework was done in the spring,
so I assume that the framing took place over the winter months,
when it was cold and snowy.

It’s conceivable that when the sun started beating down
on the porch roof in the spring, moisture trapped in the
unvented porch ceiling all winter started to “cook
off” and shrunk the 10-foot-long bottom chord of the
partial truss that makes up your porch roof. It’s
possible that the leverage exerted as it shrank flexed the
south (outboard) faces of the columns that support the porch,
and that that movement was adequate to cause a fracture at the
area of maximum extension — on the outside face of the
column but not the inside. Presumably, the north porch
didn’t react the same way because it’s not in the
direct hot sun, plus the bottom truss chord/porch ceiling is
only 6 feet long.

It’s possible, too, that the stone corners themselves are
a minor player in this scenario. I’ve been involved in
stone manufacturing, and corners are no fun to make. An outside
90-degree corner with inadequate “beef” — for
lack of a better word — is much more likely to cause
problems than a thicker product (see illustration). I’m
not intending to criticize your stone vendor, but it might not
hurt to eyeball a box of his corners and see which category
they fall into.

Also, I assume you’ve had the stone manufacturer’s
rep out to look at the job. If your installation details and
mortar mix are endorsed by the manufacturer for your climate, I
wouldn’t change your process.